This invention relates generally to telecommunications systems and more particularly to portable wireless telecommunications systems. Satellite systems exist for supporting telecommunications with fixed and mobile user terminals. Satellite based telecommunication systems, such as the Odyssey.TM. system (as proposed by one of the assignees of the present application), utilize a constellation of satellites to relay communications signals between user terminals and earth or base stations. The user terminals are assigned to earth stations. The earth stations direct calls to and from the assigned user terminals. The user terminals and associated earth stations communicate along preassigned communications channels having a preassigned bandwidth (subband) centered about a carrier frequency.
Technological advances in the last several years to satellite based systems have made it possible to offer voice and data services to hand held terminals on a global basis. The systems can also provide services to fixed installation terminals and basic telephony services in areas lacking terrestrial telecommunications infrastructure. The key objectives for these personal communication satellite services are to offer the services at lower prices to subscribers and to provide a high level of service quality. In order to provide the quality service at a low price, the number of simultaneous subscribers each satellite can support must be maximized while the capital cost of the system (including the cost of terminals) is minimized. The number of subscribers that can be supported depends on the bandwidth available for terminal-satellite communications and satellite-earth station communications, the power required by each terminal, the satellites' RF transmission capability, a variety of physical environment factors, and national/international regulatory constraints (such as terminal radiated power constraints, satellite power flux density constraints and out-of-band emissions constraints.
Several communications systems are in existence, and others have been proposed. One of these systems, the Odyssey.TM. telecommunications system (proposed by one of the assignees of this application) uses an enhanced continuous phase subclass of orthogonal direct-sequence code division multiple access (ODSCDMA), frequency orthogonality, and frequency re-use techniques to maximize user capacity. Inexpensive and portable battery-operated devices may be used in the Odyssey.TM. system and are commonly used in other systems. In addition, battery-operated devices must be designed for power efficiency to achieve a reasonable operating time.
One of the most significant factors which limits the operating time of portable wireless communications devices is the power efficiency of the transmitter amplifier. Typical non-linear amplifiers achieve a peak efficiency of approximately 55% in the saturation region versus approximately 20% in the linear region. For this reason, it is highly desirable to operate these amplifiers as close to saturation as possible to take advantage of the nearly 35% increase in efficiency.
As an additional consideration, wireless communications devices must also limit emissions outside the allocated bandwidth to prevent disrupting other services. A standard industry practice for reducing out-of-band (OOB) emissions is to combine Offset Quadrature Phase Shift Keying (OQPSK) with filtering or pulse shaping before amplification. OQPSK techniques are incompatible with the Odyssey.TM. system because the link is frequency orthogonal and not phase locked. The compromises required for OQPSK techniques are unacceptable, and both filtering and pulse shaping create a time-varying power envelope at the input to the amplifier which is undesirable when used with non-linear amplifiers.
When non-linear amplifiers are operated in saturation, they will significantly distort an input signal with time-varying power. Standard approaches for controlling OOB emissions are ineffective when used with a saturated non-linear amplifier because the amplifier will distort the signal of interest and regrow the out-of-band energy. While it is possible to place the filter after the amplifier, this would result in a significant insertion loss. Maintaining the same RF power level would require increasing amplifier power consumption, further reducing transmitter efficiency and operating time. This option is clearly impractical for battery-powered communications devices.
An example of constant-envelope modulation can be found in "SBPSK: a robust bandwidth efficient modulation for hard-limited channels," Dapper and Hill, IEEE, pages 31.6.1-31.6.6 (1984). A family of constant-envelope modulation formats in which spectral containment is achieved through the use of controlled phase trajectories is described. A BPSK code is used in a non-linear channel, and a linear interpolation is used between one phase state and the next. However, the linear interpolation causes broadening in the spectral domain which degrades frequency orthogonality. Thus, the method described by Dapper & Hill would not be compatible with modulation schemes using frequency orthogonality such as the Odyssey.TM. system.
Many different methods have been developed for utilizing the available bandwidth. Instead of using the frequency domain of a transponder, it is possible to time-share the entire bandwidth. Time division multiple access (TDMA) provides access to the available transponder spectrum on a time-shared orthogonality basis. In a simple example, one carrier fully occupies the transponder at a time. Each station transmits a burst of digitized voice, video, or data in its exclusively assigned interval. Each user goes in an assigned time slot, one at a time. A drawback of TDMA, however, is that each time slot must be allocated, whether or not it is used. As a result, certain time slots may be empty. TDMA also requires gaps between users, and one user cannot utilize another user's unused time slot to increase the efficiency of the system. Thus, many inefficiencies exist in TDMA systems.
Another method, frequency-division multiple access (FDMA), distributes the users in frequency bands. For example, one user may continuously transmit at a certain frequency and others may transmit at separate frequencies. A similar disadvantage exists in FDMA; a single user may not utilize the other frequencies of other users even if they are not using them. This also leads to inefficiencies. In code division multiple access (CDMA), each transmit station uses a unique pseudorandom code to spread its transmitted signal. Each authorized receiving station in the network must have the identical pseudorandom noise (PN) code for retrieving and collecting the information. Other networks may operate simultaneously within the same spectrum as long as different, non-interfering (orthogonal) codes are used for transmitting and receiving the signals.
Orthogonal, direct sequence CDMA is a variation of standard CDMA. Frequency orthogonality and frequency re-use techniques can also be used.
Orthogonal direct sequence refers to the fact that there is a precomputed code so that a user can be separated out. The code is known by both the handset and by the ground station. The code is necessary to separate the user information out of the data stream.
Direct sequence refers to a system that is not frequency hopping; it is not jumping all over the spectrum from frame to frame. Direct sequence is on a single frequency and a single code. The knowledge of the code is used to separate the information of the user out of the signal. Thus, orthogonal direct sequence refers to knowing the user's code, and that the user is on the same frequency all the time. This is in contrast to frequency hopping spread spectrum where the user is on one frequency, jumps to another frequency, and jumps to a third frequency to avoid something that is interfering with the user's transmission. This frequency hopping is done to avoid the source of the interference since it is most likely not interfering on every frequency.
Frequency orthogonality refers to the fact that subbands are offset at a frequency separation related to the chip rate. The frequency separation can be computed by knowing just the data rate. Thus, the next subband can be displaced by the amount of the frequency separation so that the subbands will not interfere with each other.
Frequency re-use techniques can be described as follows. Beams used in satellite transmissions are so large that they are broken down into cells. Each cell has a set of frequencies associated with it. For example, one set of frequencies can be used for a first cell. However, the same set of frequencies cannot be used for a cell adjacent to the first cell because they would interfere with each other. However, the set of frequencies used in the first cell can be re-used on another cell if it is far enough away from the first cell.
Another CDMA standard, IS95, uses either amplitude shaping or filtering or some combination of the two to vary the power of the signal in amplitude over time. Thus, the chip will be shaped so that when it passes through a filter and passes through a satellite, the data can be separated back out. However, any kind of amplitude change going through a non-linear channel, when the amplifier is heavily compressed generates regrowth from the amplifier which is very undesirable. A great deal of distortion is also created.
Standard industry practices for controlling OOB emissions with a non-linear amplifier significantly degrade system capacity and performance. Therefore, a need exists for a modulation scheme compatible with non-linear amplifiers to permit the manufacture of inexpensive, mass produced, communications devices compatible with the Odyssey.TM. telecommunications system and others.